with_sample_covariates() throws incoherent error when fetching a covariate that doesn't exist in the datastore

Witness

```r
library(FacileData)
an_fds() |> 
  samples() |>
  with_sample_covariates("beef")
```

Gives this error:

```
Error in `bind_rows()`:
! Can't combine `..1$value` <character> and `..2$value` <logical>.
Run `rlang::last_trace()` to see where the error occurred.
```

This is related to #4 